Manufacture of tufted pile carpets normally involves tufting a primary backing followed by washing, dying and drying the tufted backing and then subjecting the same to a finishing operation in which a secondary backing is laminated to a backside of the tufted primary backing.
Tufting usually is accomplished by inserting reciprocating needles threaded with yarn into the primary backing to form tufts of yarn. Loopers or hooks, typically working in timed relationship with the needles, are located such that the loopers are positioned just above the needle eye when the needles are at an extreme point in their stroke through the backing fabric. When the needles reach that point, yarn is picked up from the needles by the loopers and held briefly. Loops or tufts of yarn result from passage of the needles back through the primary backing. This process typically is repeated as the loops move away from the loopers due to advancement of the backing through the needling apparatus.
If desired, the loops can be cut to form a cut pile, for example, by using a looper and knife combination in the tufting process to cut the loops. Alternatively, the loops can remain uncut.
Primary backings for tufted pile carpets are typically woven or nonwoven fabrics made of one or more natural or synthetic fibers or yarns, such as jute, wool, polypropylene, polyethylene, polyamides, polyesters, and rayon. Films of synthetic materials, such as polypropylene, polyethylene and ethylene-propylene copolymers also can be used to form the primary backing.
The tufts of yarn inserted in the tufting process are usually held in place by untwisting of the yarn as well as shrinkage of the backing. In the finishing operation, the backside or stitched surface of the backing usually is coated with an adhesive, also commonly referred to as a backcoat, such as a natural or synthetic rubber or resin latex or emulsion or a hot melt adhesive, to enhance locking or anchoring of tufts to the backing. Use of such backcoats also improves dimensional stability of the tufted carpet, resulting in more durable carpets of improved skid and slip resistance.
Generally, the tufted carpet is further stabilized in the finishing operation by laminating a secondary backing, for example a thermoplastic film or a woven or nonwoven fabric made from polypropylene, polyethylene, or ethylene-propylene copolymers or natural fibers such as jute, to the tufted primary backing. The adhesive used in the finishing operation bonds the primary backing to the secondary backing.
In carpet lamination processes, basic requirements for adhesives include ability to bond strongly to the primary backing, the tuft stitches protruding through its backside and the secondary backing. In particular, ability to adhere to nylon and polypropylene is important because the combination of these materials (polypropylene backing fabrics and nylon face yarns) accounts for a large percentage of carpet manufactured at present. Activation temperature of a hot melt adhesive, that is, temperature at which the adhesive softens and flows sufficiently to wet and penetrate the backing surfaces and tuft stitches, must be below the temperature at which the backing and face yarns melt or suffer other damage due to heating, for example, relaxation of oriented polyolefin yarns in the backings. Adhesives also must have low enough viscosities at temperatures employed in finishing to achieve good wetting of the backings and sufficient encapsulation of tuft stitches to make the tuft yarns resistant to pull-out, pilling and fuzzing. In addition, for commercial practice, economics of a carpet manufacturing process utilizing hot melt adhesive must be at least as good as those of conventional latex lamination techniques which remain the dominant lamination process in commercial carpet manufacture.
A number of hot melt adhesives and processes using the same have been proposed for use in carpet lamination. For example, U.S. Pat. No. 3,551,231, issued Dec. 29, 1970 to Smedberg, discloses a hot melt adhesive carpet lamination process in which molten adhesive consisting of an ethylene-vinyl acetate copolymer and, optionally, waxes (e.g. microcrystalline and polyethylene waxes), fillers (e.g. calcium carbonate), resin extenders (e.g., dicyclopentadiene alkylation polymers) and antioxidant is applied to a tufted primary backing and then a secondary backing is contacted with the so-applied, molten adhesive under pressure after which the assembly is cooled to solidify the adhesive. U.S. Pat. No. 3,583,936, issued June 8, 1971 to Stahl, discloses hot melt adhesives for tufted carpet lamination comprising about 10-35 weight percent ethylene copolymer having about 60-85 weight percent ethylene units and about 40-15 weight percent lower vinyl ester, acrylate or methacrylate units; about 10-25 weight percent wax, such as microcrystalline, petroleum, polyolefin, or paraffin wax, having a melting point sufficient to give an adhesive composition with a softening point greater than 190.degree. F.; and about 50-70 weight percent resin extender composed of a base resin prepared from reactive olefins and diene monomers of 5-7 carbons and substantially free of polymerized aromatics and a modifying resin of certain low molecular weight dicyclopentadiene alkylation polymers. Optionally, such adhesives also include fillers, antioxidants, pigments and plasticizers. U.S. Pat. No. 3,900,361, issued Aug. 19, 1975 to Hoppe et al., discloses fusion coating masses suitable for use in carpet backing lamination in which an atactic homo- or copolymer of butene-1 is used to impart flexibility to adhesives comprising a natural or synthetic resin or wax, plasticizer, filler, elastomer and stabilizers.
As disclosed in such patents, an adhesive in molten form is applied to a backing material. Another backing material is brought into contact with the adhesive under pressure, melting and subsequent cooling of the adhesive serving to bond the backing materials. Application of molten adhesive typically is performed using applicator rolls, such as those used in latex lamination processes, that pass through a bath of molten adhesive or by extrusion of molten adhesive onto a backing. The large, heated vessels or extruders required for handling and application of hot melt adhesives in molten form are not needed in latex lamination processes; accordingly, conversion of conventional latex processes to use of hot melt adhesives in molten form can require substantial capital investment.
As an alternative to carpet lamination processes in which hot melt adhesives are applied in molten form, U.S. Pat. No. 3,734,800, issued May 22, 1973 to Ryan, discloses forming hot melt polymers or other thermoplastics into continuous sheet or film and directing the same between primary and secondary backings, heating the backings and adhesive in contact to melt the adhesive and then solidifying the adhesive to form a high strength laminate. According to the patent, advantages of the process reside in elimination of the need for liquids in the lamination process and ability to utilize existing latex lamination ovens for melting the adhesive.
Use of adhesive films to laminate unwoven tapes for other applications also is known from U.S. Pat. No. 3,734,812, issued May 22, 1973 to Yazawa, disclosing use of thermoplastic films, such as low density polyethylene of low molecular weight, ethylene-vinyl acetate copolymer, ethylene acrylamide copolymer and polypropylene, to laminate stretched, unwoven tapes of polymeric materials to form perforated structures useful for protecting agricultural products from animals, birds and insects, for fishing, as a curtain or upholstery material or a bag for vegetables, cereals or powders.
U.S. Pat. No. 3,940,525, issued Feb. 24, 1976 to Ballard, discloses use of thermoplastic films, such as polyethylene, polypropylene or ethylene-vinyl acetate copolymer films, as secondary backings in carpet lamination, the films being laminated to tufted primary backings with hot melt adhesive applied in molten form.
U.S. Pat. No. 4,434,261, issued Feb. 28, 1984 to Brugel et al., discloses extrudable, self-supporting hot melt adhesive sheets containing ethylene-vinyl acetate or other ethylene copolymers, certain plasticizers, fillers and other additives for use in laminating materials such as spun bonded polyester and polypropylene. Use in carpet manufacture is not disclosed.
Another problem with hot melt adhesive carpet lamination methods has been ineffective distribution of adhesive into the secondary backing, rather than into face yarn tuft stitches on the underside of the primary backing. This occurs because the secondary backing generally heats more rapidly than the primary backing and tuft stitches during the lamination process either as a result of direct contact between the secondary backing and the heat source or heated surfaces in the process or the thermal insulating effect of the tufts on the primary backing or a combination of these factors. In turn, the hot melt adhesive activates more rapidly in the vicinity of the secondary backing such that the adhesive tends to flow toward that backing in preference to the primary backing. This preferential flow toward the secondary backing may be enhanced when that backing is more porous than the primary backing, for example when the primary backing is tightly woven or has a high density of tuft stitches and the secondary backing is loosely woven. Such a distribution of the hot melt adhesive results in incomplete tuft encapsulation which, in turn, results in poor carpet wear characteristics. Delamination strength and tuft bind strength also are sacrificed and adhesive is effectively wasted due to ineffective distribution of adhesive within the structure.
From U.S. Pat. No. 3,684,600, issued Aug. 15, 1972 to Smedberg, it is known to apply a low viscosity precoat composition in molten or solution form to a primary backing prior to backcoating with hot melt adhesive. The precoat is used in an amount sufficient to bond the tuft stitch fibers, thereby enhancing bonding of the primary and secondary backings and yielding fuzz-resistant carpets. A variety of precoat adhesives is disclosed including, for example, polyethylene, polypropylene, polybutene, polystyrene, polyesters and ethylene-vinyl acetate copolymers. A precoat blend of ethylene-vinyl acetate copolymer with waxes and a resin mixture of polyethylene, microcrystalline wax, alkyl aromatic thermoplastic resin and unsaturated aliphatic thermoplastic resin also is disclosed. U.S. Pat. No. 4,552,794, issued Nov. 12, 1985 to Goss, also discloses precoat compositions for use in carpet lamination. According to that patent, an improved precoat composition has a Brookfield viscosity of about 25 to about 500 centipoise at about 150.degree. C. and comprises, by weight, about 1 percent to about 19 percent low density polyethylene; about 0.1 percent to about 5 percent of a nitrogen-containing silane cross-linking compound; about 65 percent to about 85 percent of a resin; about 5 percent to about 7 percent of a hydrocarbon wax; and, optionally, up to about 30 percent of a naphthenic oil. Application of precoat in molten form is disclosed.
While precoat hot melt adhesives have been proposed to improve tuft stitch encapsulation, application thereof in molten form creates additional expense and complexity in the lamination process by requiring additional materials, process steps and equipment.
Thus, there remains a need for a hot melt adhesive carpet lamination process that will provide tufted carpets of good bond strength between primary and secondary backings as well as good tuft stitch encapsulation and tuft bind strength that can be practiced without substantial alteration of equipment used in conventional latex lamination processes.
It is an object of this invention to provide an improved hot melt carpet lamination process and adhesives therefor. A further object of the invention is to provide a hot melt adhesive carpet lamination process in which use of molten adhesives and equipment for application thereof is eliminated. A further object is to provide composite hot melt adhesives for such a process and components of such composite adhesives.
I have now found that these objects can be achieved through the use in carpet lamination of composite hot melt adhesive in sheet form comprising layers of adhesive compositions to overcome the unfavorable temperature-viscosity gradient that exists in the carpet structure during the lamination operation. A lower viscosity, resin-rich adhesive in sheet form is applied on the stitched side of a tufted primary backing to promote good tuft encapsulation and tuft-bind strength. A higher viscosity, filler-rich barrier sheet adhesive is placed next to the secondary backing to prevent bleedthrough of the lower viscosity adhesive into the secondary backing during the lamination process. By applying the composite adhesive in sheet form, use of liquids and equipment required for application of adhesives in molten form is eliminated.